Fluidisation of yield stress fluids under vibration*

We compare the fluidisation of sessile drops of molten chocolate and Carbopol which are both shear-thinning fluids with a similar yield stress, but exhibit very different elastic properties. We find that these materials spread when the forcing acceleration exceeds the same threshold imposed by the initial deposition process. However, the height reduction due to spreading is typically at least three times greater in chocolate than in Carbopol for similar forcing. The two materials exhibit very different spreading behaviours with the Carbopol drop performing large-amplitude oscillations with the forcing frequency, both above and below the threshold. We investigate these viscoelastic oscillations and provide evidence of complex nonlinear viscoelastic behaviour in the vicinity of the spreading threshold. In fact, above the fluidisation threshold, our drop automatically adjusts its shape to remain at the yield stress.